The present invention relates to a microwave circuit, and more specifically to the improvement in a microwave circuit having a three plate construction.
A so-called microwave plane circuit consists of a microwave circuit which is formed on a dielectric substrate. Being also called a microwave integrated circuit (MIC), the microwave circuit is recently finding extensive applications to meet the demand for constructing circuits having small size, reduced weight and a high packing density. Types of transmission lines used for the MIC can be classified into microstrip lines, coplanar lines, and three plate lines. Among the lines of these types, the microstrip lines are most widely used for the MIC's, in which a grounded conductor is formed on one surface of a dielectric substrate, and transmission circuits and other circuits are formed on the other surface. The microstrip lines have an advantage in that they can be easily formed. However, the surface on which the transmission circuit is formed, is left open and affected by the cover of a case in which it is contained. Moreover, microstrip lines have large radiation losses. The same holds true for coplanar lines. On the other hand, three-plate lines can be formed with difficulty to some extent. A three-plate line is a strip line sandwiched between two plate-line grounded conductors. In this case, however, since the transmission circuit is sandwiched by grounded conductors, radiation losses are small. Further, the transmission circuit is little affected by the cover of the case in which it is contained.
FIG. 1 is a perspective view illustrating the construction of a conventional generally employed three plate line, in which reference numerals 1 and 2 denote grounded conductors, 3 and 4 denote dielectric boards, and 5 denotes a central conductor.
FIG. 2 is a section view along the line II--II of FIG. 1. Although the central conductor 5 shown here is a single transmission line, it may be a microwave circuit of any form. Thus, with the three plate line, the electromagnetic waves do not leak since the central conductor 5 is sandwiched by the grounded conductors 1 and 2. Therefore, there is no radiation loss, and the circuit is not affected by external disturbance. Accordingly, the three plate line has small mutual interaction relative to external circuits, and is best suited for constructing MIC's.
FIG. 3 is a section view of a conventional example, which corresponds to FIG. 2, and in which two transmission lines 51 and 52 run in parallel. When a plurality of circuits are formed in the same substrate, however, mutual interaction takes place between the neighboring circuits (the mutual interaction develops not only in the three plate lines but also in the plane circuits, in general). When either one of the circuits, between which the mutual interaction is taking place, is an amplifier circuit, coupling is formed between the input portion and the output portion of the amplifier circuit via the neighboring circuit, giving rise to the occurrence of oscillation. To remove mutual interaction, it had hitherto been attempted to increase the distance between the neighboring circuits. To increase the distance among the circuits, however, inevitably results in the decrease of the integration degree of the whole MIC, which prevents the production of small sized MIC's.